The Coronavirus Disease 2019 (COVID-19) was first reported in December 2019 and was classified as a pandemic by the World Health Organization on March 11, 2020. Wuhan was the Chinese city that showed the first symptoms of Covid-19 and had been under lockdown measures from January 23 until April 8, 2020. During the first 2 months after the city’s reopening, there were only a few sporadic COVID-19 cases in Wuhan (six newly confirmed cases from April 8 to May 10, 2020).
However, there was still concern about the risk of COVID-19 in Wuhan, which seriously affected the operation of industrial production and social services, and impeded the normal lives of residents. In order to ascertain the current status of the COVID-19 epidemic, the city government of Wuhan carried out a comprehensive citywide nucleic acid screening of SARS-CoV-2 infection from May 14, 2020, to June 1, 2020. There were 10,652,513 eligible people aged 6 years old and older in Wuhan (94.1% of the total population). The nucleic acid screening was completed in 19 days (from May 14, 2020, to Jun 1, 2020), and tested a total of 9,899,828 persons from the 10,652,513 eligible people (participation rate, 92.9%). Of the 9899,828 participants, 9,865,404 had no previous diagnosis of COVID-19, and 34,424 were recovered COVID-19 patients. The screening of the 9,865,404 participants without a history of COVID-19 found no newly confirmed COVID-19 cases and identified 300 asymptomatic positive cases with a detection rate of 0.303 out of 10,000. Of the 300 asymptomatic positive cases, two cases came from one family and another two were from another family. There were no previously confirmed COVID-19 patients in these two families. A total of 1174 close contacts of the asymptomatic positive cases were traced, and they all tested negative for the COVID-19. There were 34,424 previously recovered COVID-19 cases who participated in the screening. Of the 34,424 participants with a history of COVID-19, 107 tested positive again, giving a positive rate of 0.310%.
What this research suggests
Virus cultures were negative for all asymptomatic positive cases, indicating no “viable virus” in positive cases. All asymptomatic positive cases and their close contacts were isolated for at least 2 weeks until the results of nucleic acid testing were negative. None of the detected positive cases or their close contacts became symptomatic or newly confirmed with COVID-19 during the isolation period. Existing laboratory virus culture and genetic studies showed that the virulence of SARS-CoV-2 virus may be weakening over time, and the newly infected persons were more likely to be asymptomatic and with a lower possibility of having the virus than earlier infected cases.
In summary, the detection rate of asymptomatic positive cases in the post lockdown Wuhan was very low (0.303/10,000), and there was no evidence that the identified asymptomatic positive cases were infectious. These findings enabled decision-makers to adjust prevention and control strategies in the post- lockdown period.
If you are interested, you can find more about the research in the article “Post-lockdown SARS-CoV-2 nucleic acid screening in nearly ten million residents of Wuhan, China”. https://www.nature.com/articles/s41467-020-19802-w
